Cancer is one of the most life threatening diseases in which cells in a part of the body experience out-of-control growth. According to the latest data from American Cancer Society, cancer is the second leading cause of death in the United States (second only to heart disease) and claimed more than 550,000 lives in 2009. In fact, it is estimated that 50% of all men and 33% of all women living in the United States will develop some type of cancer in their lifetime. Therefore cancer constitutes a major public health burden and represents a significant cost in the United States. For decades, surgery, chemotherapy, and radiation were the established treatments for various cancers. Patients usually receive a combination of these treatments depending upon the type and extent of their disease. But the chemotherapy is most important option for cancer patient when the surgery treatment is impossible.
Nitrogen mustards, a kind of classic DNA alkylating agents, were among the first chemotherapeutic agents rationally applied to the treatment of cancer. Mechlorethamine, an analogue of mustard gas and derived from chemical warfare research during World War II, has been used in the cancer chemotherapy for over 60 years. Nitrogen mustards generally exert cytotoxic activity by forming DNA adducts or crosslinks between DNA strands under conditions present in cells, directly interfering with the reproductive cycle of the cell. The following are the structures of some well-known nitrogen mustards.

Melphalan is a well-known DNA alkylating nitrogen mustard approved for the treatment of multiple myeloma (Musto P, et al, Expert Opin Investig Drugs, 2007, 16(9):1467-87). Melphalan, in combination with Prednisone (MP), has been used as first line standard therapy for decades for elderly multiple myeloma patients ineligible for autologous stem cell transplantation. Currently, MP is still the backbone of new first-line MM chemotherapeutic regimens such as MP-thalidomide (MPT), MP-lenalidomide (MPR), and MP-bortezomib (MPV). In addition, the use of Melphalan alone as a conditioning regimen for autologous stem cell transplant is considered “Standard of Care” for multiple myeloma treatment. As for today, melphalan is in 196 active clinical trials for a variety of caner indications, such as multiple myeloma, leukemia, lymphoma, MDS, ovarian cancer, breast cancer, and brain tumor, etc.
Bendamustine, first synthesized in 1963, consists of an alkylating nitrogen mustard moiety and a purine-like benzimidazol moiety with a suggested purine-analog effect (Barman Balfour J A, et al, Drugs 2001; 61: 631-640). Bendamustine has been shown to have substantial activity against low-grade lymphomas (Herold M, et al., Blood, 1999; 94, Suppl 1: 262a), multiple myelomas (Poenisch W, et al., Blood 2000; 96, Suppl 1: 759a), and several solid tumors (Kollmannsberger C, et al., Anticancer Drugs 2000; 11: 535-539). It was also reported that bendamustine effectively induces apoptosis in lymphoma cells (Chow K U, et al., Haematologica, 2001; 86: 485-493). On March 2008, the FDA granted approval to market bendamustine for the treatment of chronic lymphocytic leukemia (CLL). On October 2008, the FDA granted further approval to market bendamustine for the treatment of indolent B-cell non-Hodgkin's lymphoma (NHL) that has progressed during or within six months of treatment with rituximab or a rituximab-containing regimen. Currently bendamustine is in clinical trials for a variety of caner indications, such as leukemia, lymphoma, small cell lung cancer, multiple myeloma, MDS, ovarian cancer, breast cancer, and brain tumor.
The nitrogen mustard Cyclophosphamide remains one of the most successful and widely utilized antineoplastic drugs in modem cancer therapy (Emadi A, et al, Nat Rev Clin Oncol. 2009 November; 6(11):638-47). Cyclophosphamide is an inactive prodrug that requires enzymatic and chemical activation and the resultant nitrogen mustard produces the interstrand and intrastrand DNA crosslinks that account for its cytotoxic properties. Cyclophosphamide based chemoregimens such as FCR, FCE, AC, and R-CHOP remains the cornerstone of first-line treatment for breast cancer, lymphoma, CLL, ovarian cancer, and soft tissue sarcomas.
Although the conventional DNA alkylating nitrogen mustards have made a significant contribution to cancer treatment, they have major limitations. As we know, the conventional DNA alkylating nitrogen mustards will damage the DNA and then the cellular DNA damage response pathway will be activated to arrest cell cycle progression, induce apoptosis, and repair the DNA damage. However, cancer cells treated with the conventional nitrogen mustards may easily escape from the cell cycle arrest and apoptosis, and may repair the DNA damage efficiently, leading to quick development of drug resistance and treatment failure. Therefore, it is urgent to continuously search in this field of art for the new generation nitrogen mustards with significantly improved anti-cancer activities.
Recently year, Cyclin-dependent kinases (CDK) has recently emerged as an important disease target for cancer treatment (Marcos Malumbres et al, Nat Rev Cancer. 2009 March; 9(3):153-66; Silvia Lapenna, et al, Nat Rev Drug Discovery, 2009 July; 8(7):547-66). CDKs are a family of serine/threonine kinases that regulate key cellular processes including cell cycle progression and RNA transcription (Shapiro G I. J Clin Oncol. 2006 Apr. 10; 24(11): 1770-83). Heterodimerized with regulatory cyclin units, CDKs can be generally divided into two groups based on their functions. The first group consists of core cell cycle components and governs the cell cycle transition and cell division: cyclin D-dependent kinases 4/6 and cyclin E-dependent kinase 2, which control the G→S transition; cyclin A-dependent kinases 1/2, a critical regulator of S-phase progression; cyclin B-dependent CDK1, required for the G2→M transition; and cyclin H/CDK7, the CDK-activating kinase. The second group, so called transcriptional CDKs, includes cyclin H/CDK7 and cyclin T/CDK9 which phosphorylate the C-terminal domain (CTD) of RNA polymerase II and promote transcriptional initiation and elongation.
The deregulation of the CDK activity is detected in virtually all forms of human cancer, most frequently due to the overexpression of cyclins and loss of expression of CDK inhibitors (de Career G et al, Curr Med Chem. 2007; 14(9):969-85). CDK4/6 inhibition has been shown to induce potent G1 arrest in vitro and tumor regression in vivo (Lukas J et al., Nature. 1995 Jun. 8; 375(6531):503-6; Schreiber M et al., Oncogene. 1999 Mar. 4; 18(9):1663-76; Fry D W et al., Mol Cancer Ther. 2004 November; 3(11): 1427-38). Various approaches aimed at targeting CDK2/1 have been reported to induce S and G2 arrest followed by apoptosis (Chen Y N et al., Proc Natl Acad Sci USA. 1999 Apr. 13; 96(8):4325-9; Chen W et al., Cancer Res. 2004 Jun. 1; 64(11):3949-57; Mendoza N et al., Cancer Res. 2003 Mar. 1; 63(5):1020-4) Inhibition of the transcriptional CDKs 7 and 9 can affect the accumulation of transcripts encoding anti-apoptosis family members, cell cycle regulators, as well as p53 and NF-κB-responsive gene targets (Lam L T et al., Genome Biol. 2001; 2(10):RESEARCH0041). All these effects contribute to the induction of apoptosis and also potentiation of cytotoxicity mediated by disruption of a variety of pathways in many cancer cell types (Chen R et al., Blood. 2005 Oct. 1; 106(7):2513-9; Pepper C et al. Leuk Lymphoma. 2003 February; 44(2):337-42). CDKs are therefore recognized as an attractive target for the design and development of compounds that can specifically bind and inhibit the cyclin-dependent kinase activity and its signal transduction pathway in cancer cells, and thus can serve as therapeutic agents. As today, there is a list of CDk inhibitors, (e.g. AT-7519, AZD5438, Flavopiridol, P1446A-05, P276-00, CYC202, SCH 727965, BAY 1000394, LEE011, etc) currently in clinical trials for treatment of cancer.